A hydraulic tensioner has been widely used in a timing belt or a timing chain, which transmits rotation between a crankshaft and a camshaft of a vehicle engine, to suppress vibration generated during displacement of the timing belt or timing chain, and to maintain proper tension. A number of conventional hydraulic tensioners have been proposed which feature a check valve, as disclosed in Japanese Laid-Open Patent Publication No. Hei-11-336855.
FIG. 8 shows an example of such a conventional hydraulic tensioner 1 having a check valve. The hydraulic tensioner 1 is attached to an engine body on a slack side of a chain 3 trained over a drive sprocket 101, which is rotated by a crankshaft of the engine and driven side sprockets 102, each fixed to a camshaft.
In this hydraulic tensioner 1, a plunger is protruded and retracted from the front surface of the housing 2, and by pushing the back surface of a lever 105, which is near the pivoting end and pivotably supported on the engine body side, with the plunger 3, tension is imparted to the slack side of the chain 103 through the lever. Further, on the tension side of the chain 103, a guide 106, which guides the chain 103, is attached to the engine body. Further, when the drive sprocket 101 is rotated in a direction of the arrow, the chain 103 is displaced in the direction of the arrow. Consequently, the rotation of the drive sprocket 101 is transmitted to the driven sprockets 102.
In a hydraulic tensioner 1 shown in FIG. 9, a plunger 3 is slidably received in a hole 2a formed in a housing 2. A hollow bore 3a having an open end is formed in the plunger 3. A plunger biasing spring 4 inside the bore 3a biases the plunger 3 toward the open end of the plunger receiving hole 2a. The plunger biasing spring 4 is always biased so that the front end of the plunger 3 is protruded outside the plunger receiving hole 2a. A high pressure chamber 5 comprises the plunger receiving hole 2a and the hollow portion 3a. The high pressure chamber 5 is filled with oil supplied by an oil supply source not shown. The oil is supplied through a check valve 6, which is described later.
The check valve 6, which faces the hollow bore 3a of the plunger 3, causes oil to flow into the high pressure chamber 5 and blocks the back flow of oil out of the chamber. The check valve 6 is incorporated into the plunger receiving hole 2a formed in the housing 2. The check valve 6 comprises a ball seat 7, a check ball 8 facing the ball seat 7, a cylindrical coil spring 9, which push-biases the check ball 8 against the ball seat 7, and a retainer 10, which retains the cylindrical coil spring 9 and limits movement of the check ball 8, as shown in FIG. 9.
The ball seat 7 forms a valve seat 7a at an end surface facing the check ball 8. Oil passages 7b, which communicate with an oil supply source not shown, are formed in the ball seat 7. The ball seat 7 is press-fitted into a bottom portion in the plunger receiving hole 2a of the housing 2. The cylindrical coil spring 9 is shown in FIG. 10 in an enlarged view. A brim 10a is formed on one end of the retainer 10 and a communicating hole 10b through which oil is passed is formed on a side of the retainer 10. Further, in the retainer, a protrusion 10c is formed which protrudes toward the check ball 8. The protrusion limits the distance “S” through which the check ball 8 is moved so that the coil spring 9 is not bottomed out and over-stressed, so as to exert pressure on the wall of the retainer. The brim 10a of the retainer 10 is retained against the bottom of the plunger receiving hole 2a by the plunger biasing spring 4, so that the check ball 8 is pushed against the valve seat 7a of the ball seat 7 by the cylindrical coil spring 9. The retainer 10 is produced by sheet-metal working using a mold.
In the hydraulic tensioner 1 formed as mentioned above, the lever 105 exerts an impact force F on the front end of the plunger 3 in response to a change of tension in the chain 103. When the plunger 3 is rapidly pushed back while resisting a biasing force of the plunger biasing spring 4, the pressure of oil in the high pressure chamber 5 is increased to push the check ball 8 onto the valve seat 7a of the ball seat 7 as shown in FIG. 9, whereby the back flow of oil from the high pressure chamber 5 into the oil passage 7b of the ball seat 7 is prevented.
As a result, the oil pressure in the high pressure chamber 5 is further increased, and oil leaks through a small gap formed between an outer circumferential surface of the plunger 3 and an inner circumferential surface of the hole 2a. The oil leaks through the gap and is discharged outside the housing 2. Accordingly, the impact force F acted on the plunger 3 by fluid resistance due to the viscosity of oil, which is generated at the time of the discharge of oil, is reduced and the vibration of the plunger 3 due to said impact force F is speedily diminished.
On the other hand, when the engine is started, and the chain 103 is momentarily slackened on the hydraulic tensioner 1 side, the plunger 3 is momentarily protruded from the housing 2 in a direction of the arrow in FIG. 11 by the biasing force of the plunger biasing spring 4. This advances the lever 105 against the chain 103 to tension the chain and remove the slack.
In this case, since the oil pressure in the high pressure chamber 5 is decreased, the check ball 8 in the check valve 6 is separated from the valve seat 7a of the ball seat 7. The check valve 6 is thereby opened, and oil is supplied into the high pressure chamber 5 from the oil passage 7b in the ball seat 7. At this time, the check ball 8 moves while pressing the cylindrical coil spring 9. However, the distance “S” through which the check ball 8 moves is limited by a protrusion 10c provided in the retainer 10, so that the cylindrical coil spring does not bottom out or become over-stressed.
However, hydraulic tensioner 1 has the following problems. That is, since as mentioned above, the protrusion 10c provided in the retainer 10 limits the movement of the check ball 8, the molding of the protrusion 10c requires working accuracy. Further, when the protrusion 10c wears, close contact of the cylindrical coil spring 9 with the retainer 10 is encountered and the cylindrical coil spring 9 is broken. To improve the wear resistance of protrusion 10c, heat treatment is required for increasing the hardness of the surface of the protrusion 10c, which increases the production cost. Alternatively, if the height of the protrusion 10c is increased, a time margin can be provided for an amount of wear till the occurrence of close contact of the cylindrical coil spring 9 with the retainer 10. However, providing the amount of wear with a time margin was impossible for reasons of molding, strength and limitation of inner diameter of the cylindrical coil spring 9.